This invention relates to electro-optic measuring devices, and in particular to a means and a method for measuring voltage by electro-optic response.
Researchers have longed to develop a device which uses electro-optic response to measure voltage. However, researchers have been unable to develop a practical way to measure voltage without using a complex device and a computer for purposes of interpreting electro-optic data to determine an unknown voltage. In 1987, the Electric Power Research Institute (EPRI) issued a report, prepared by the National Bureau of Standards, titled Optical Power Line Voltaqe and Current Measurement Systems, Volumes I and II, September 1987, which indicates that standard current and voltage transformers are reliable measuring devices but are expensive and even dangerous when adapted for measuring high voltages. Volume I of the Report describes an example of an electro-optic voltage sensor. An electrooptic crystal has an unknown electric field applied to it. The crystal also receives circularly polarized light traveling in a direction perpendicular to the field and outputs the light with a relative phase shift due to the birefringent effect of the electric field on the light. The amount of relative phase shift of the "cross-plane" component (i.e., the light component in a plane transverse to the electric field) with respect to a "parallel plane" component (i.e., the light component in a plane parallel to the "stress axis" i.e. the electric field) is related to the strength of the electric field.
The Report does not suggest any simple way to process the light output from the electro-optic element to determine the electric field and thus the voltage. The Report discusses several practical problems including thermal and piezoelectric effects that make accurate voltage measurement very difficult. Accordingly, the Report indicates that electro-optic devices are not yet practical. For example, a high DC component of the electric field and thermal and piezoelectric effects may create a much greater relative phase shift than the AC component of the field. The high DC component cannot be measured easily with limited gain and thus AC cannot easily be measured. Development of a practical electro-optical measuring device would be useful for achieving virtually instantaneous measurement across a wide band of response. Moreover, electro-optic measurement is substantially free of any electrical interference. Accuracy can be quite high in spite of thermal effects. In fact, the Report states that accuracy can be near 1% even at temperatures over 100.degree. C., and that by using temperature compensation accuracy can be improved by a factor of ten. Such a device would have a low cost, and be quite versatile, as optical devices can also measure temperature, pressure and even perform chemical analysis of gases.